Although this website aims to promote the Astro Navigation Demystified series of books, it is hoped that it will also provide a useful resource for navigators, scholars and students of the subject.

A wealth of information on the subject of astro navigation can be found under the various headings on the menu bar at the top of the page and in the archives listed down the right. The images below give links to various pages which may be of interest.

Why Astro Navigation? There is rapidly growing interest in the subject of astro navigation or celestial navigation as it is also known. It is not surprising that, in a world that is increasingly dominated by technology and automation, there is an awakening of interest in traditional methods of using the celestial bodies to help us to navigate the oceans.

Astro navigation is not just for navigators; the subject is an interwoven mix of geography, astronomy, history and mathematics and should appeal to both mariners and scholars alike.

Russia is one of the few countries in the world to acknowledge the educational value of astro navigation and to include it as an important part of the school curriculum. In other countries, institutions such as nautical schools and maritime colleges include the subject in their curricula as a subject in its own right while for some independent schools, it provides the perfect theme for integrated studies and open-ended project work.

The question is often asked: ‘how could seafarers navigate the oceans if the global positioning system (GPS) failed? The answer is quite simple; they could revert to the ‘fail-safe’ art of astro navigation. The problem here though, is that we have become so reliant on automated navigation systems that traditional methods are being forgotten. Even so, there is a very real danger that the GPS could be destroyed. During periods of increased solar activity, massive amounts of material erupt from the Sun. These eruptions are known as coronal mass ejections and when they impact with the Earth they cause disturbances to its magnetic field known as magnetic storms. Major magnetic storms have been known to destroy electricity grids; shut
down the Internet, blank out communications networks and wipe out satellite systems
(including the global positioning system).

Couple this danger with that posed by cyber terrorists who could block GPS signals at any time, then it can easily be seen that navigators who rely solely on electronic navigation systems could be faced with serious problems.

Unfortunately, many sea-goers are deterred from learning astro navigation because they perceive it to be a very difficult subject to learn. In fact, it is very interesting and easy to learn but sadly, some writers and teachers of the subject attempt to disguise its simplicity by cloaking it in an aura of mystery.

email: astrodemystified@outlook.com

Jeremiah Dixon and Charles Mason plotted the famous Mason Dixon Line in 1765, long before the days of GPS or any other electronic navigation equipment. How was it then that they were they able to fix positions from the midst of the forests of the Iroquois?

They would not have been able to survey the land using triangulation methods because suitable landmarks would have been hidden by the trees. They would not have been able to measure the altitude of celestial bodies because there would not have been a visible horizon. All they would have been able to see would be a small circle of sky through the canopy above them and therein lies the clue.

They used an instrument known as a zenith sector which is a fixed vertical telescope through which an observer is able to view a small circle of sky centred at the zenith of his geographical position. By using this device, they were able to accurately measure the zenith distance of celestial bodies that came within the telescope’s field of view.

In the diagram below, Z marks the zenith of the observer, X is the position of a celestial body and O is the Earth’s centre. The zenith distance is the angular distance ZX which is subtended by the angle XOZ . In other words it is the angular distance from the observer’s zenith to the celestial body. (For a fuller explanation of zenith distance follow this link:)

By measuring the zenith distance of a celestial body at the instant that it crosses the observer’s meridian, the observer is able to determine the latitude of his position because the zenith distance will be equal to the distance from the latitude of the geographical position of the body to the latitude of the observer in nautical miles measured north or south (click here for an explanation of this).

Mason and Dixon plotted their line in this way choosing stars whose declinations were close to the latitude 39o 43′ N, the east/west boundary between Pennsylvania and Maryland. Because thechronometer had not yet been invented, they were not able to calculate longitude which partly explains why their line ran along a parallel of latitude.

They chose to use only stars for their observations because the declination of a star changes very slowly and can be considered to be fixed for short periods of time. Furthermore, the magnification of the zenith sector telescope is far greater than the telescope of a sextant and so they were able to use many faint stars that we would not normally be able to use for navigation.

To establish a north/south boundary they would have followed a line bearing true south or true north from a known landmark such as a hill or small town. It is interesting to note that the majority of the boundaries between American states, which were established before the advent of the chronometer, also ran east/west or north/south.

The boundary between Delaware and Pennsylvania, which was also fixed by Mason and Dixon, involved mainly conventional surveying techniques because it followed an arc known as the ‘twelve mile circle’ around the town of New Castle. Similarly, the Delaware-Maryland boundary was based on conventional surveying because it was designed to bisect the Delmarva Peninsular instead of following a meridian.

If we wish to use a celestial body’s position in the celestial sphere to help us to navigate on the Earth’s surface, we must be able to translate that body’s celestial co-ordinate position into a geographical position expressed in terms of our earthbound geographical co-ordinate system.

As we discussed in the previous post, we express a celestial body’s position in the celestial sphere in relation to its angular distance east or west of the celestial meridian that passes through the ‘First Point of Aries’. Similarly, in the geographical co-ordinate system, we express a position on the Earth’s surface in relation to its angular distance east or west of the Greenwich Meridian. The ‘Declination’ of a body expresses its angular distance north or south of the Celestial Equator in the same way that we use latitude to define a position north or south of the Equator.

The following explanation requires a little imagination. In this diagram, the Greenwich Meridian is projected onto the Celestial Sphere.

Point X denotes the position of star Acamar in the celestial sphere and PRP1 represents the meridian running through the position of Acamar.

The Sidereal Hour Angle (SHA) is the angular distance from the meridian of the First Point of Aries to the meridian of the celestial body (R) measured westwards.

The Greenwich Hour Angle of Aries (GHA Aries) is the angular distance, measured westwards, from the projected Greenwich Meridian to the meridian of the First Point of Aries.

The Greenwich Hour Angle of Acamar (GHA Acamar) is equal to the sum of the SHA Acamar and the GHA Aries.

GHA Aries to the nearest second can be interpolated from tables in the Nautical Almanac as can the SHA and Declination of the Navigational Stars. Accordingly, for the example below, the following values have been taken from Nautical Almanac:

Longitude of the Geographical Position of Acamar. The GHA is equivalent to the longitude; however, we must remember that GHA is measured westwards from the Greenwich Meridian from 0o to 360o whereas Longitude is measured either east or west from Greenwich from 0o to 180o. Therefore, in this case, since the GHA of Acamar is greater than 180o, the longitude will be East so we must subtract it from 360o to convert it to an easting as follows: Longitude = 360o – 341o 59’.80 = 18o 00’.2 East.

Declination of Acamar = S40o 14′.3 (Note that the declination of the stars can be regarded as constant and so no further calculation is necessary).

Latitude of the Geographical Position of Acamar. Since the declination is equivalent to the latitude, we can state that the latitude of the GP is 40o 14′.3 South.

We can now state that we have translated the celestial co-ordinates of Acamar from SHA:315o 20’.50, Declination:S40o 14′.3 to a Geographical Position of 18o 00’.2 East, 40o 14′.3 South.

Note. It is not necessary for the navigator to calculate the GHA of the Sun, Moon and planets because the Nautical Almanac tabulates these for you.

Using The Geographical Position (GP) Of A Celestial Body To Determine Our Own Position. By measuring the altitude of a celestial body, we are able to calculate the zenith distance which will give us the distance in nautical miles from the observer’s position to the geographical position of the body. The azimuth will give us the direction of the GP of the body from the observer’s position. This explains why measuring the altitude and azimuth are the first steps in determining our position in celestial navigation. Those who wish to learn how we do this can follow this link to find a brief explanation: Astro Navigation in a Nutshell. However, ‘Astro Navigation Demystified’ contains a more comprehensive explanation.

In astronomy, we need a celestial co-ordinate system for fixing the positions of the celestial bodies in the celestial sphere.

We express a celestial body’s position in the celestial sphere in relation to its angular distance north or south of the Celestial Equator and east or west of the celestial meridian that passes through the ‘First Point of Aries’.

Declination. The Declination of a celestial body is its angular distance north or south of the Celestial Equator. The declinations of the stars change very slowly and can be considered to be almost constant for up to a month at a time. The declination of the Sun changes relatively fast from 23.43o N. to 23.43o S. and back again during the course of a year. The Moon’s declination is more difficult to predict because the rate of change is even more rapid than that of the Sun and the pattern of the changes is less uniform. The declinations of the planets are complicated by the facts that they are at varying distances from the Sun, have different orbital patterns and travel at different speeds.

Declination can be summarised as the celestial equivalent of latitude since it is the angular distance of a celestial body north or south of the Celestial Equator.

Note. The latitude of the tropic of Cancer is currently drifting south at approximately 0.5’’ per year while the latitude of the tropic of Capricorn is drifting north at the same rate.

The First Point of Aries is usually represented by the ‘ram’s horn’ symbol shown on the left. Just as the Greenwich meridian has been arbitrarily chosen as the zero point for measuring longitude on the surface of the Earth, the first point of Aries has been chosen as the zero point in the celestial sphere. It is the point at which the Sun crosses the celestial equator moving from south to north (at the vernal Equinox in other words). The confusing thing is that, although this point lay in the constellation of Aries when it was chosen by the ancient astronomers, due to precession, it now lies in Pisces.

Note. Because of the difficulty of inserting the symbol for Aries into text we substitute it with the character Y in the text below

Right Ascension (RA). This is used by astronomers to define the position of a celestial body and is defined as the angle between the meridian of the First Point of Aries and the meridian of the celestial body measured in an easterly direction from Aries. RA is not used in astro navigation, Sidereal Hour Angle is used instead.

Sidereal Hour Angle (SHA). This is similar to RA in as much that it is defined as the angle between the meridian of the First Point of Aries and the meridian of the celestial body. However, the difference is that SHA is measured westwards from Aries while RA is measured eastwards. This is illustrated in the following diagram:

X is the position of a celestial body in the celestial sphere.

R is the point at which the body’s meridian crosses the celestial equator. PXRP1 is the meridian of the celestial body.

is the First Point of Aries. PYP1 is the meridian of the First Point of Aries.

The Sidereal Hour Angle is the angle YPR. That is the angle between the meridian running through the First Point of Aries and the meridian running through the celestial body measured at the pole P. It can also be defined as the angular distance YR, that is the angular distance measured westwards along the Celestial Equator from the meridian of the First Point of Aries to the meridian of the celestial body.

Right Ascension can also be defined as the angle between the meridian of the First Point of Aries and the meridian of the celestial body but the difference is that it is measured in an easterly direction from Aries.

From this, we can conclude that

RA = 360o – SHA and

SHA = 360o – RA.

In Astro Navigation, we make use of our knowledge of the positions of the celestial bodies to help us to navigate on the surface of the Earth. However, in order to do so we must first relate their positions in the celestial sphere to positions on the Earth’s surface. The next post in this series explains how this is done.

A more detailed treatment of this topic can be found in Astro Navigation Demystified’.

Imagine that you are driving through Birmingham when, suddenly, your ‘Sat Nav’ starts to tell you that you are in Manchester. In such a situation, you would quickly realise that the GPS had gone haywire; however, if you were in a ship, out of sight of and beyond radar contact of land, it would not be immediately obvious that you were being given false positions. If you are one of those people who depend heavily on the GPS and believe that it will never let you down, then you might be in for a nasty shock. The New Scientist reports that Russia may be experimenting with methods of interfering with GPS signals and that these methods could quite easily be copied by other organisations including rogue nations and terrorists.

Sebastian Anthony talks of our terrifying reliance on GPS and our need to develop back-up systems. Imagine the devastating effects that a GPS failure would have on land, air and sea navigation, air traffic control, communications, power grids, radar, defence and a host of other systems very few of which have ‘back-ups’ in place.

Things can easily go wrong with the GPS even without malicious interference. For example, magnetic storms can put power grids out of action, blank out communications systems including the internet and destroy satellites (including those that serve the GPS).

I warned of these dangers on this website in 2008 with my post Could The GPS Fail when I made the point that fortunately, when it comes to navigation at sea, we do have a back-up system; a system which has been tried and tested over hundreds of years; of course I speak of Astro / Celestial Navigation.

It is all very convenient to find our way by GPS but what would we do without it when we are far out to sea where there are no roads, signposts or other landmarks to guide us? Prudent navigators keep up their skills in astro / celestial navigation by taking at least one astro fix a day when on passage. The reason they do this, is not only to practise their skills but also to keep a check on the GPS. In fact, many experienced yachtsmen and women do not employ GPS at all when on ocean passage but rely solely on their skills in astro / celestial navigation instead.

If Astro / Celestial Navigation is new to you or you just want to brush-up your skills, you might be interested in the following.

If two ships are both positioned exactly on the Equator but are separated by 900 nautical miles in an east/west direction, then in terms of longitude, they will be 15o apart and in terms of Greenwich Mean Time, they will be 1 hour apart. If however, they both move to latitude 50oN maintaining a distance of 900 nautical miles between them, they will find that their difference in longitude has increased to 23o.36. and their difference in GMT has increased to 1.5 hours.

What are the reasons for these differences?

Longitude and Distance at the Equator. The Earth’s equatorial circumference is 21639 n.m. Since the Equator is a great circle, 1o will subtend an arc of: 21639 ÷ 360 = 60.1 » 60 n.m. There are 360 meridians of Longitude so it follows that, measuring from the Earth’s centre, the angular distance between adjacent meridians at the Equator is 1o. Since, as calculated above, 1o subtends an arc of 60 n.m. it follows that the distance between adjacent meridians of longitude at the Equator is 60 n.m.

Returning to the original question, the distance between the ships when measuring along the Equator, is 900 nautical miles. Since, as explained above, the distance between adjacent meridians of longitude at the Equator is 60 n.m, the difference in longitude between the ships must be 900 ÷ 60 = 15o.

Longitude and Mean Time. The Mean Sun completes its 360o revolution of the Earth in 24 hours. So, in 1 hour, the Mean Sun moves 15o,

in 4 minutes, it moves 1o,

in 1 minute it moves 15′,

in 4 seconds it moves 1′.

From this, it becomes obvious that there is a direct relationship between arc and mean time such that 1 minute of time equals 15 minutes of arc. We know that the angular distance between meridians of longitude at the Equator is 1o and that 1o equates to 4 minutes of time so we can conclude that the mean time difference between adjacent meridians of longitude is 4 minutes.

Why Greenwich Mean Time. Greenwich Mean Time (GMT) is the local mean time anywhere on the meridian of Greenwich. Since the Greenwich meridian is used as the base meridian from which the longitude of all places on Earth are identified, it follows from the discussion above, that GMT provides the link between the longitude of a place and the longitude of Greenwich. Therefore, if we know the longitude of a position on the Earth’s surface, we can easily calculate the GMT at that position since 1o of longitude equates to 4 minutes of GMT. Alternatively, we can calculate the longitude of a place if we know the GMT there.

Returning to the original question again, we have calculated that the ships are separated by 15o of longitude which in terms of GMT, equates to a time difference of one hour.

Note. Universal Time (UT). The term Universal Time was adopted internationally in 1928 as a more precise term than Greenwich Mean Time, because GMT can refer to either an astronomical or a civil day. However, the term Greenwich Mean Time persists in common usage to this day and is generally considered to be synonymous with the term Universal Time. It should be noted that the Nautical Almanac and other tables of astronomical data usually refer to UT instead of GMT.

Longitude and Distance Along a Parallel of Latitude. The diagram below shows that, as the meridians of longitude move away from the Equator, they draw closer together until they eventually converge at the poles.

In the next diagram, the arcs AC and BD lie on different meridians of longitude. The arc AB is the distance between these meridians measured along the surface of latitude 50oN and the arc CD is the distance between the same meridians measured along the Equator. Clearly, the distance CD is much greater than the distance AB.

To Calculate The Distance Between Two Meridians Along A Parallel Of Latitude. The following formulae are used for calculating the difference in distance along a parallel of latitude (Ddist) corresponding to a difference in longitude (Dlong) and vice versa. The formulae are simply stated below without explanation but if you wish to see a full explanation of their derivation then click here).

Ddist = Dlong x Cos Lat. and Dlong = Ddist ÷ Cos Lat.

To return to the original question once again, we know that the two ships are separated by a distance of 900 nautical miles along the surface of the parallel of latitude 50oN. Using the formulae given above, we calculate the difference between them in terms of longitude as follows: Dlong = Ddist ÷ Cos Lat = 900 ÷ Cos(50) = 900 ÷ 0.64278 = 1400′.168 = 23o.33

To calculate the difference between them in terms of GMT we proceed as follows: 23o.33 ÷ 15 = 1.5 hours (since 15o of arc equates to 1 hour of mean time).

A more detailed treatment of this topic can be found in the books listed below.

In the diagram above, the celestial sphere is drawn in the plane of the observer’s meridian with the observer’s zenith (Z) at the top.

Point O represents both the observer and the Earth.

Z represents the observer’s zenith.

X is the position of a celestial body in the celestial sphere.

A is the point where the virtual circle running through the position of the celestial body meets the celestial horizon.

P and P1 are the north and south poles respectively.

Zenith. The Zenith is an imaginary point on the celestial sphere directly above the observer. It is the point where a straight line drawn from the geocentric centre of the Earth, through the observer’s position and onwards, intersects with the celestial sphere.

The Zenith Distance. The Zenith Distance is the angular distance from the zenith to the celestial body measured from the Earth’s centre. It is the angular distance ZX which is subtended by the angle XOZ and is measured along the vertical circle that passes through the celestial body. (A vertical circle is a great circle that passes through the observer’s zenith and is perpendicular to the celestial horizon).

The Altitude. Altitude is the angle AOX, that is the angle from the celestial horizon to the celestial body and is measured along the same vertical circle as the zenith distance.

Relationship Between Zenith Distance And The Nautical Mile. An angle of 1 minute at the earth’s centre will subtend an arc of length 1 n.m on the earth’s surface. Therefore if the angle XOZ is 30o (the arc ZX) will be equal to 30 x 60 = 1800 arc minutes at the earth’s surface and so the zenith distance will be equal to 1800 nautical miles.

Relationship between Altitude and Zenith Distance. Since the celestial meridian is another vertical circle and is therefore, also perpendicular to the celestial horizon, it follows that angle AOZ is a right angle and angles AOX and XOZ are complementary angles. From this we can deduce that:

Zenith Distance = 90o – Altitude

and Altitude = 90o – Zenith Distance

In the PZX Triangle diagram which is shown below, the arc AU is the arc joining the observer’s position to the geographical position of the celestial body. This arc when projected onto the celestial sphere forms the arc ZX which is the zenith distance. Therefore, from the discussion above, it can be seen that the angular distance ZX is equal to the angular distance AU which when converted to nautical miles will give us the distance from the GP of the body to the position of the observer.

Azimuth. The angle PZX is the azimuth of the celestial body and is the angular distance between the observer’s celestial meridian and the direction of the position of the body (GP).

Summarizing The Role Of Altitude, Azimuth And Zenith Distance In Celestial navigation. The preceding discussion illustrates the importance of altitude and azimuth in celestial navigation. It can be seen that by measuring the altitude of a celestial body, we are able to easily calculate the zenith distance which will give us the distance in nautical miles from the observer’s position to the geographical position of the body. The azimuth will give us the direction of the GP from the observer’s position. This explains why measuring the altitude and azimuth are the first steps in determining our position in celestial navigation.

For students of astro navigation, the various definitions of azimuth, azimuth angle and bearing can cause much confusion. It is hoped that the following will help to clarify this topic.

Bearing is the direction of something in relation to a fixed point. A bearing can be measured in degrees in any direction and in any plane.

For example, in marine navigation, relative bearing is measured in the horizontal plane in relation to the ships heading from 0o to 180o to either port (red) or starboard (green). For example, the relative bearing of an object on the port beam would be Red 90o and an object on the starboard bow would be Green 45o.

Note. The definition given above is according to the Admiralty Manual of Navigation. However, it is acknowledged that there are other definitions which state that relative bearings are measured from ‘right ahead’ in a clockwise direction from 0o to 360o. It is little wonder that there is so much confusion surrounding this topic.

Azimuth is a specific type of bearing which measures the direction of an object in relation to true north, in the horizontal plane, clockwise from 0o to 360o. For example, in terms of azimuth, due east is 090o and due west is 270o.

Azimuth is measured by use of either a magnetic compass or a gyro compass. A gyro compass is a form of electrically driven gyroscope which measures azimuth in relation to true north. Such an azimuth measurement is known as true azimuth. A magnetic compass employs a magnetised needle which is used to measure the azimuth of an object in relation to magnetic north. Magnetic compass readings must be corrected for variation and deviation in order to convert them to true azimuth.

Azimuth Angle (see diagram above). Another area of confusion is the difference between azimuth and azimuth angle. In astro navigation, when we calculate the azimuth of a celestial body, the result is expressed as an azimuth angle. Azimuth angle is measured from 0o to 180o either westwards or eastwards from either north or south. If the observer is in the northern hemisphere, the azimuth angle is measured from north and if in the southern hemisphere, it is measured from south. For example, if the true azimuth of an object is 225o, the azimuth angle for an observer in the northern hemisphere will be N135oW but for an observer in the southern hemisphere, it will be S045oW.

To Convert Azimuth Angle to True Azimuth. The rules for converting azimuth angle to true azimuth are summarised in the following table:

In the two previous posts of this series, we focussed on astro navigation aspects of the ancient voyaging techniques of the Polynesians and the Micronesians. Because of that narrow focus, we did not do justice to the abounding knowledge and skills of these amazing navigators. Now, we will try to redress the balance by retelling, in more detail, the story of the voyage from Palmyra Atoll to Hawaii which was outlined in the post ‘Pillars of the Sky’. For the sake of simplicity, in telling this story, we will use the nautical and geographical terminology used today instead of struggling to find the equivalent ancient Polynesian terms.

It is the year 1400 and Alaka’i, an experienced and highly respected Hawaiian navigator is sailing Manu, a Polynesian voyaging canoe, home to Hawaii after a visit to Tahiti, the ancient homeland of his people. The distance from Tahiti to Hawaii is over 2000 nautical miles so, along the way, Alaka’i has used several small islands and atolls as ‘waypoints’ to rest the crew and to replenish their supplies of food and water. The atoll that we now know as Palmyra Atoll is about two thirds of the way from Tahiti to Hawaii and is the last of these waypoints.

Alaka’i uses the time at the atoll to prepare for the last leg of the journey to Hawaii. He has no compass, no chart and none of the hydrographical paraphernalia that we have today. However, he does have his own home-made ‘star compass’ and in his mind, he has a plan of mathematical perfection involving hundreds of complex calculations, intuitively made without formal mathematical training or any form of mathematical notation. He unconsciously uses these calculations to constantly visualize the position of his craft and in this way, he practices ‘dead reckoning’ in his head instead of on a chart. His calculations involve many factors including speeds; distances; wind and leeway; current set and drift; points of sail; courses and headings; wave patterns and shapes; swells and swell deflections and the directions of the Sun and stars.

He is able to recognise many stars along with their parent constellations and he knows the ‘on top’ stars for the major islands. The star compass shows the rising and setting points of important stars and this enables him to choose appropriate‘steering stars’to suit his course. The star compass also shows the rising and setting points of the Sun at the equinoxes and the solstices. It is shortly before the Summer Solstice and Alaka’i knows that the Sun will be to the north of Hawaii at this time of the year.

Alaka’i selects a prominent position on the atoll and from this position, at midday when the Sun is at its zenith, he scratches a line on a rock pointing towards it to indicate the direction of North. When the Sun is very high in the sky, as it is at noon, it is very difficult to judge its direction but Alaka’i has a method to overcome this problem. He takes a piece of semi-transparent cloth which is stretched over a bamboo frame; he uses this as a filter to enable him to look at the Sun and point a finger towards it. He then lowers his hand to indicate the Sun’s direction on the horizon.

At sunrise and sunset, he makes further scratches on the rock to indicate the rising and setting points of the Sun. When this is done, he places the star compass on the rock and aligns the mark for Polaris to the scratch mark showing the direction of the midday Sun. He notices that the scratch marks for the rising and setting points of the Sun coincide roughly with the rising and setting points of Altair on the star compass and he will use this knowledge to help to orientate the canoe to its course during the voyage.

Arcturus is the ‘Star on Top’ for Hawaii and Alaka’i knows, that when he sees the star is immediately above him at its zenith, he will be on the same latitude as the island but he won’t know whether he is to the east or the west of it. If he were to be in that situation, he could easily sail off in the wrong direction and then have to spend days or possibly weeks sailing back and forth until he found land or else become hopelessly lost. His solution to this problem is to deliberately steer to one side of the island so that he will know in which direction to turn.

He knows that, in the Summer, the direction of the winds in the region of Hawaii fluctuate between NE and ENE for 90% of the time and that the current sets in the same direction. If he deliberately steers to the west of the island, he will face the laborious task of beating against the wind to reach land. He decides therefore, to aim for a point to the east of Hawaii which will enable him to sail downwind and so make a ‘windward landfall’, a technique that was described in the post‘Pillars of the Sky’.

Hawaii lies roughly 870 nautical miles away in a direction between Polaris and the rising point of the star Kochab (approximately 008o). However, because he aims to be upwind of Hawaii, he plans to make for a point 100 miles further to the east. He calculates the course to this imaginary point to be in the direction midway between Kochab and Dubhe (roughly 015o) and he makes a further scratch on the rock to mark this.

The final task in Alaka’i’s preparations is to set up transit marks to help him to orientate the canoe to the planned course when he sails from Palmyra. He instructs crewmen to cut two tall poles and directs them to stand them in the ground in line with the scratch mark that indicates the course.

Alaka’i sails Manu from Palmyra in an ENE breeze using the transit poles to set the canoe on course. He watches the transit poles closely to help him to gauge the set and drift of the current which he judges to be setting East at about half a knot. He observes that the waves are steeper and taller than could be accounted for by the wind speed and this confirms for him that the current is setting in the opposite direction to the wind. By observing the wake and bow-wave he estimates that the speed through the water is approximately 4 knots. He also observes the angle between the wake and the fore and aft line of the boat to estimate that the leeway caused by the wind is approximately 5 degrees.

Without the aid of navigation charts and relative velocity diagrams, he takes all of these factors into account to intuitively calculate a course to steer and he puts the canoe on a close reach, sailing on the starboard tack with a heading of NbE (approx. 010o)

Alaka’i is aware that swells, created by distant winds, may travel in a completely different direction to waves that are driven by local winds and that they do not change direction frequently in the way that local winds and waves do. He notes that the swell is coming from just abaft the port beam so he instructs the helmsman to maintain this orientation in order to keep the canoe on the correct heading. He does not orientate the canoe by the local wind and waves because he knows that these can change direction quickly. At midday, he is pleased to see that, when the Sun is at its zenith, its direction is about 10o on the Port bow which tells him that the helmsman is keeping the canoe on the correct heading of 010o.

Alaka’i consults the star compass to devise a plan to keep Manu orientated to the correct course by day and by night. He calculates that the Sun should be about 20o ahead of the starboard beam at sunrise; 10o on the port bow at midday and on the port beam at sunset. During darkness, Polaris should be about 10o on the port bow; the rising point of Dubhe should be about 15o on the starboard bow and its setting point 35o on the port bow. Altair should rise about 10o ahead of the starboard beam and set abeam to port. Acrux will rise on the starboard quarter and will slowly lead the Southern Cross westward over the southern sky and then set again on the port quarter.

Kingman Reef is a hazardous, partly submerged reef which lies 36 nautical miles northwest from Palmyra Atoll. Alaka’i is confident that his course will take him well clear of the reef but even so, he is constantly on the alert for signs to warn him of the danger.

Kingman reef does not host land based birds and it is too low to produce the cloud effects referred to in Pillars of the Sky but Alaka’i has another trick up his sleeve for detecting land that may be over the horizon. Sometimes a swell may not be visibly detected but the Polynesian voyagers developed techniques for sensing weak swells, often by lying in the bottom of the boat and feeling for faint variations in its movement. Because they are no longer driven by the winds that created them, swells are easily deflected by land and if these deflected swells can be detected, they may indicate, to a skilled Polynesian navigator, that land is nearby. Alaka’i uses this ability to sense deflected swells to ensure that he does not sail too close to Kingman Reef.

By nightfall, Manu is well clear of Kingman Reef and Alaka’i is able to relax. Before lying down to get some well earned sleep, he instructs the helmsman to hold the course by keeping the canoe on a close reach on the starboard tack with Polaris about 10o on the port bow. He does not sleep for long because, as well as checking the bearings of Kochab, Dubhe, Altair and Acrux when they rise and set, he needs to begin to observe the star Arcturus at its zenith so that he will know when he has reached the latitude of Hawaii. (Arcturus is the ‘star on top’ for Hawaii).

Alaka’i knows that, at about 200 miles north of Palmyra atoll, the set of the current will change from easterly to westerly as Manu leaves the ocean current that we know as the Equatorial Counter Current and enters the North Equatorial Current. As expected, during the morning of the third day out from Palmyra Atoll, Alaka’i notices turbulence in the water and confused wave shapes and patterns from which he deduces that he has entered the region between the two currents. Towards the end of the morning, the current is setting westwards with a drift that Alaka’i knows from experience, will be about half a knot. The wind is ENE (about 070o) but its speed has increased to about 15 knots causing a leeway angle of roughly 10o. Alaka’i takes these new factors into account and in an effort to keep to the planned course, he calculates that the course to steer should be towards the rising point of Dubhe (025o). In an effort to keep to the new course to steer, he sails the canoe as close to the wind as possible so that it is now close-hauled on the starboard tack. At midday, Alaka’i is relieved to see that the Sun is about 25o on the port bow and this tells him that Manu is sailing in the direction of Duhbe’s rising point and so he is confident that the helmsman is keeping the canoe on the correct heading.

During the night of the ninth day of the voyage, Alaka’i observes that Arcturus is immediately above him at its zenith; this tells him that he has reached the latitude of Hawaii and so he alters course to the west in order to make a ‘windward landfall’. However, he knows that Hawaii Big Island is the southernmost of the Hawaiian islands and he doesn’t want to risk missing it altogether by steering too far south. With this in mind, he adjusts the heading to WNW so that, with Maui and the other Hawaiian islands strung out in a line to the northwest, he will have a block of islands to aim for.

He orientates the canoe by keeping the North Star just before the starboard beam in order to maintain his WNW course during the night. He knows that Arcturus will set right ahead on this course which is convenient because he can use it as the ‘steering star’ as well as the ‘on top star’. With the wind from the NE, he puts the canoe on a broad reach, sailing on the starboard tack. With the wind and current helping them, he calculates that Manu is making about 8 knots over the ground and he hopes to sight land before the end of the next day.

At dawn on the tenth day, Alaka’i observes that the Sun is on the starboard quarter when it rises at EbN and this tells him that Manu is on course. At noon, one of the crewmen excitedly points to a number of boobies and frigate birds hovering over a shoal of fish. This is encouraging news for Alaka’i because he knows that the maximum range of these birds from land is around 50 to 60 miles. At about this time, long white clouds can be seen just above the horizon. Alaka’i believes that these are the cloud-trails which appear during the day as the heat of the land forces moist sea air up over Hawaii and Maui’s high volcanoes. From experience, he knows that, in good visibility, the cloud-trails can be seen over 50 miles away and he is now certain that land is over the horizon.

Towards late afternoon, large numbers of birds can be seen including noddies and white terns which are normally found within 20 miles of land. He can now make out the silhouette of the volcanoes against the Sun which is beginning to set. The distance to the horizon from a voyaging canoe is around 10 miles and because he cannot yet see the land beneath the volcanoes, he judges that the shore-line is about 20 miles distant.

He is wary of approaching a lee shore in darkness so at dusk, he decides to heave-to for the night. With the current and the wind as they are, he estimates that the canoe will drift towards the islands at about one knot at the most and that it will not reach land before dawn. When Arcturus reaches its zenith during the night, Alaka’i observes that it is still immediately overhead and this tells him that the canoe is keeping to the latitude of Hawaii.

As day breaks on the eleventh day, there is great excitement amongst the crew as the shoreline of Hawaii can now be seen about 6 miles ahead. Alaka’i steers Manu towards Waiakea Bay on Hawaii Big Island, navigates her through a small channel in the reef and heads for his home village of Waiakea.

The last post in this series discussed the Polynesian’s Star Compass and their use of ‘steering stars’ for direction finding. Although the Polynesians made extensive use of a form of dead reckoning to estimate position, as far as we know, they did not have a method of fixing a vessel’s position at sea. If they did, we will never know because their methods were closely guarded secrets which were known only to elite groups of navigators and were never recorded. However, David Lewis, in his book ‘We The Navigators’ discusses how early Polynesian navigators pin-pointed the position of certain islands by what they called the ‘Star on Top’.

If the latitude of a certain island coincides with the declination of a star, it stands to reason that when the star crosses the meridian of that island, it will be immediately above it. So, to an observer on the island, the star will be overhead when it reaches its zenith; in other words, it will be the ‘Star on Top’. It was believed that the ‘on top’ stars for all the islands were held up in the sky by pillars and that the sky was supported by these pillars which were known as ‘Pillars of the Sky’.

So, how would the Polynesian navigators have used an island’s ‘star on top’ to help them to navigate towards it? Let’s try an example:

Palmyra is a tropical Atoll located roughly half way between Hawaii and Samoa. Although it has no indigenous population now, it may well have had in the past and its position would have made it a suitable waypoint for voyages between Hawaii and the South Pacific islands, particularly since it has an abundant supply of fresh water, coconut palms, many species of nesting birds and lagoons teeming with fish. It is highly likely therefore, that ancient Polynesian wayfarers would have made voyages between Palmyra and Hawaii in their large outrigger canoes. We will use such a voyage for this example.

When Polynesian exploration was at its height around one thousand years ago, the declination of Arcturus was just to the north of Hawaii but due to precession, it slowly moved south and now sits above the southern tip of Hawaii (19.2oN). So, for many centuries, Arcturus would have been the ‘star on top’ for Hawaii and could still serve that purpose today.

Hawaii lies about 1,000 miles to the north of Palmyra and so the obvious course to steer from Palmyra to Hawaii would seem to be north. However, for two reasons this would not have been the chosen course.

Firstly, for the first part of the voyage, the Equatorial Counter Current would set the canoe eastwards, but from about half way, the North Equatorial Current would set it westwards again. Assuming that the voyage was conducted during Hawaii’s summer months, the prevailing Trade Winds would also set the vessel westwards. So, taking the winds and currents into account, the navigator would have to lay off a course to the east of North.

The second reason is this. If the navigator sailed north until Arcturus was directly overhead at its zenith, all he would be able to tell from this would be that his canoe was on the same latitude as Hawaii but he would not know if he was to the east or the west of the island. If his chosen course caused the boat to finish downwind of the island, he would then have the difficult task of beating against the wind and tide to reach his goal. However, if he if he deliberately steered a course that would take him upwind of the islands, he would then be able to sail downwind while maintaining latitude by keeping Arcturus ‘on top’. This technique of deliberately steering so as to finish upwind of the target island was called ‘Windward Landfall’.

A star compass which shows the rising and setting points of the stars which would likely to be of help for a voyage from Palmyra to Hawaii has been constructed below.

The navigation plan for the first part of the voyage would probably be to sail from Polymyra on a course of roughly NNE keeping Polaris on the port bow with the Southern Cross on the starboard quarter during the hours of darkness. (Note that the Southern Cross is not circumpolar north of 34o South and that Acrux, its brightest star, would rise at roughly SSE). Rigel Kentaurus and Hadar in the constellation Centaurus rise on approximately the same bearing as Acrux but shortly after it. These two stars are known as the ‘Pointers’ because an imaginary line from Rigel Kentaurus to Hadar will point towards the Southern Cross.

The direction from which the North East Trade winds blow fluctuates between NE and and ENE so with any luck, the canoe would be able to complete the whole of this leg of the journey on the starboard tack without the need to beat upwind. The heading would be checked by aligning the canoe with the star Dubhe when it rose at approximately NNE. (Dubhe is in the constellation Ursa Major (Great Bear) and is not circumpolar south of 38o North). At the time of aligning the canoe with Dubhe, the angle between the direction of the advancing waves and the fore and aft line of the canoe would be noted and the navigator would use this information for guidance during daylight hours.

When a point was reached where Arcturus (declination 19.2oN) was immediately overhead at its zenith, the course would be changed to westerly for the second part of the voyage to sail downwind to Hawaii.

You will see from the star compass that the stars Alnilam and Altair set approximately due west and so they would make suitable ‘steering stars’ for this part of the voyage. Alnilam is a winter star and Altair is a summer star so one of them will always be visible at night. The plan for the second part of the voyage would probably be to use Alnilam or Altair as the ‘steering star’ while keeping Polaris on the starboard beam and the Southern Cross to port. The latitude of Hawaii would be maintained by keeping Arcturus ‘on top’.

Quite how one can tell what point is ‘immediately overhead’ from a canoe which is rocking and rolling in a choppy sea is not clear but according to Lewis, the Polynesian navigators had several secret methods such as lying in the bottom of the canoe facing upwards. Another method was the ‘floating cane’ which Lewis vaguely describes. Apparently, a cane would be cut below two consecutive growth rings so that a short length of cane which was sealed at one end and open at the other was obtained. A small weight would be attached to the sealed end and the cane would then be filled with water. In theory, when the cane was placed in a container of water, it would remain vertical in spite of the movement of the canoe.

Probably, for Polynesian navigators, the first indication that the canoe was approaching land would be the sighting of certain birds such as terns, noddies, boobies and frigate birds which are land-based and therefore fly out from the land in the mornings and return to it in the evenings thereby giving the navigator indications not only of the nearness of land but also its direction. Pelagic species such as the albatross which roam freely over the open ocean would obviously be of no navigational use and so it would have been important to have the ability to recognise the different species.

Clouds would have been another indication of approaching land. David Lewis gives an in-depth discussion of ‘cloud lore’ which was developed by Polynesian sailors over many centuries. There is no space here to discuss this topic fully but there are a few useful tips that modern day navigators could take from the Polynesians. Firstly, although an island may be below the horizon, clouds above it may be visible. Drifting clouds tend to slow down and ‘stick’ over an island for a while and then pick up speed again. Some islands, particularly those with mountains or volcanoes will often appear to have a permanent cloud above them as moist air rising above them condenses and then evaporates again as it descends.

*Ref. Lewis, David, 1972. ‘We the Navigators’, Honolulu: The University Press of Hawaii.

Updated version.

“Know the stars and you will always have a compass” (The Revenant) *

Nainoa Thompson tells us how that, for centuries before European sailors reached the Pacific Ocean, the South Sea Islanders accurately found their way from island to island without the aid of magnetic compasses, sextants or any other navigational equipment. They navigated the Pacific by using their knowledge of natural phenomena such as the directions of the winds and waves, the flight paths of birds, cloud formations and the colour of the sea. Their most important technique however, was the ‘Star Compass’ which was not a physical tool but a mental construct based on their knowledge of the directions that certain stars would rise and set.

Such knowledge was acquired over many hundreds of years and passed down by word of mouth and example with each generation committing it to memory. It is a tragedy that because the navigational knowledge and techniques of the early Polynesians and Melanesians were not recorded, they have largely been forgotten or would have been if it were not for the enthusiastic work of Nainoa Thompson, Mau Piailug and others.

In modern times, the majority of navigators rely on GPS to find their way although many still keep the traditional art of Astro (Celestial) Navigation alive. How many could navigate the oceans without either of these methods though?

It is well known that we could be deprived of the GPS at any moment for many reasons including the ever increasing threat of cyber attack, coronal mass ejections, equipment failure or shipwreck. How would we cope in such situations? Would it not be a good idea to construct and then memorise our own star compasses for the latitudes at which we sail? This would not be as difficult as it may seem at first. Let us try an example:

Sailor Jerry plans to conduct an experiment by sailing from Fogo Island, Newfoundland (49.8oN) to Mullion Cove (50.02oN) in the British Isles without the aid of GPS or any other navigational equipment, not even a magnetic compass (no jokes about rum line sailing please). He plans to to sail due East along parallel 50oN using only a star compass until he makes landfall, hopefully at one of the following: the Scilly Isles (49.9oN), Lands End (50.1oN) or Lizard Point (49.95oN). Once he has made landfall, he will be able to set a course for Mullion Cove.

Jerry is aware that strong ocean currents in the North Atlantic will make it difficult for him to stay on track; at the beginning of his voyage, he will be pushed south by the Labrador and Irminger currents, later he will be pushed east and then northeast by the North Atlantic Current and then he may get caught by the Canary Current which will push him south east. To add to his difficulties, he knows that although the star compass will provide him with directional information, it will not help him to find his position. To overcome these problems so that he can try to stay on track, he plans to frequently calculate his latitude from the North Star by the method explained here. It is likely that he will not have a sextant in a survival situation so he plans to measure the altitude of the North Star using a home made clinometer for that purpose.

The method he uses to construct the star compass is simple. Firstly, he needs to select several bright stars which he could use to guide him in the right direction (preferably, stars with a magnitude of 1 or less). The lower the star is to the horizon, the better it is to indicate direction so his next step is to calculate the azimuth of the chosen stars when they rise and set from his latitude of 50oN.

He doesn’t have time to observe and memorise the directions of the stars as the South Sea Islanders did so he allows himself the luxury of a nautical almanac to help him with this task.The following table shows the data that Jerry collected to help him to construct his star compass. Notes: 1. To make his calculations of azimuth, he uses the method explained here).

2.To calculate the azimuth of a star when it is on or just above the horizon, Jerry uses an LHA of approximately 271o for dawn and 089o for dusk. (The method of calculating a star’s LHA is shown at step 1 here).

3. Bearings of the Sun at sunrise and sunset are included in the table but only for the equinoxes and the solstices since these are the only four dates on which we can be sure of the Sun’s declination without the aid of an almanac. This information is not included in Jerry’s star compass on this occasion because he will be making the voyage in August.

Below is a diagram of the star compass that Jerry constructed. Because he wants to steer an easterly course, he has selected stars that rise between northeast and southeast and set between northwest and southwest. The North Star (Polaris) and Kochab in Ursa Minor (Little Dipper) are also shown to help him find the direction of north.